The conversion of fossil fuels such as coal, natural gas and petroleum coke to liquid hydrocarbon fuels and/or chemicals has been the subject of intensive research and development throughout the industrialized world for many years to provide a practical alternative to petroleum crude oil production and open-up the world's vast reserves of coal as a competitive source for essential hydrocarbons. Many processes have been developed for the direct or indirect catalytic hydrogenation of fossil fuels to yield liquid hydrocarbons; some large pilot plants have been built and operated, and about twenty commercial scale plants have been built for the conversion of coal to primarily liquid hydrocarbons. Of these twenty plants, most were built by the German government during World War II. About half of them were built using the well-known Fischer-Tropsch process for converting synthesis gas to liquid hydrocarbons in contact with iron catalyst and, operationally at least, worked well enough for war-time needs. Subsequently, the South African Government (SASOL, Ltd) built commercial size coal conversion plants to produce hydrocarbon fuels and chemicals which also were successfully based on indirect conversion using Fischer-Tropsch chemistry and iron catalysis.
From an operational point of view, the commercial liquefaction of coal or natural gas based on indirect Fischer-Tropsch (F-T) chemistry has been demonstrated to be an engineering success. However, true economic success has so far eluded the developers of direct or indirect coal or natural gas liquefaction processes, largely because of the low cost of crude oil as the competitive alternative but also because of the high cost of the direct or indirect liquefaction conversion step where process economic performance is so dependent on feed price, catalyst cost, activity, resistance to attrition and other challenges which must be overcome or ameliorated by cost-enhancing process modifications. There is a genuine potential for indirect coal or natural gas liquefaction via Fisher-Tropsch (F-T) chemistry to substantially narrow the competitive gap between that process and crude oil. A key to that potential is improvement in the chemistry of catalysis as applied to the Fischer-Tropsch process for synthesis gas conversion to hydrocarbons.
A known, practical method for preparing liquid hydrocarbons rich in valuable 1-olefins is to convert a fossil fuel, especially natural gas, to synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, by steam reforming of natural gas followed by conversion of the synthesis gas to liquid hydrocarbons over a precipitated iron F-T catalyst. However, most precipitated iron catalysts in the F-T process are especially fragile and break down easily in slurry-phase reactors into very fine particles under conventional reaction conditions. A significant portion of the hydrocarbon products comprise waxy hydrocarbons and these waxy materials become mixed with sub-micron size iron catalyst particles forming the slurry to the extent that separation of the very fine catalyst particles from the entraining waxy product of the F-T process is extremely difficult. Costly and comparably complicated separation processes must be resorted to in the conventional F-T process to effect the separation of the fine precipitated iron catalyst particle carry-over from the wax product of the F-T process. The result is a substantial loss of recyclicable precipitated iron catalyst particles coupled with an added cost burden on the F-T process economics from the additional cost of replacing lost catalyst and the more elaborate process steps used to try to separate fine catalyst particles from wax products. Catalyst/wax separation difficulties have been a major barrier for precipitated iron catalysts to be successfully applied in commercial operation.
In applicants' U.S. Pat. Nos. 6,265,451 and 6,277, 895, incorporated herein by reference in their entirety, skeletal iron F-T catalysts are taught for the production of liquid hydrocarbons in a slurry reactor from fossil-fuel derived synthesis gas. The patents teach and claim a relatively simple and inexpensive method for preparing the skeletal iron F-T catalyst that experiences less attrition, easy catalyst/wax separation, and the conversion of syngas is higher than that obtained by using fused iron as catalyst. Also, the conversion of the feed is equivalent to that achieved by precipitated iron F-T catalysts. Product selectivity favors the production of distillate hydrocarbons and less wax.
Now, the applicants herein come forward with the discovery of a fully integrated process that successfully utilizes the many advantages of the foregoing skeletal iron F-T catalyst in a slurry bed reactor and overcomes the major process limitations experienced heretofore in the production of liquid hydrocarbons from synthetic gas from fossil fuels such as natural gas and coal.